This relates to systems and methods of cast molding toric contact lenses.
Contact lenses having a toric optical zone (commonly referred to as “toric contact lenses”) are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides a cylindrical optical correction to compensate for the astigmatism. Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. The toric surface may be formed in either the posterior lens surface (back surface toric lens) or in the anterior lens surface (front surface toric lens).
Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have a stabilization feature to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism when the lens is worn. For example, a section of the lens may be thicker or thinner than another section to provide the stabilization. Toric contact lenses are manufactured with a selected relationship (or offset) between the cylindrical axis of the toric optical zone and the axis of the stabilization feature. This relationship may be expressed as the number of degrees (rotational angle) that the cylindrical axis is offset from the stabilization feature.
Accordingly, toric contact lenses, similar to spherical contact lenses, may be offered with a range of different spherical powers. Toric contact lenses, however, are also offered with a range of cylindrical optical corrections, and with a range of cylindrical axis orientations in order to accommodate patients with different astigmatic conditions. For example, a typical toric contact lens prescription will designate spherical correction, cylindrical optical correction and cylindrical offset.
One method for manufacturing contact lenses is cast molding. Cast molding of contact lenses involves depositing a curable mixture of polymerizable monomers in a mold cavity formed by two mold sections, curing the monomer mixture, and disassembling the mold assembly and removing the lens. One mold section forms the anterior lens surface (anterior mold section), and the other mold section forms the posterior lens surface (posterior mold section).
Prior to the cast molding of the contact lens, each of the mold sections is formed by injection molding a resin in the cavity of an injection molding apparatus. Mounted in the injection molding apparatus are tools for forming the optical surface on the mold sections. Whereas the mold sections are typically used only once for casting a lens, the injection molding tools are used to make many molds. The tools have optical quality molding surfaces so that the mold sections being made therefrom also have optical quality lens-molding surfaces.
U.S. Pat. No. 5,611,970 (Apollonio et al.) and U.S. Pat. No. 6,491,393 (Appleton et al.) provide good background of cast molding toric contact lenses. The methods disclosed in these patents are particularly useful in that the same types of anterior and posterior mold sections can be aligned at multiple rotational positions. Accordingly, the same types of anterior and posterior mold sections can be used to mold toric lenses with the same spherical and cylindrical optical corrections but with different cylindrical axis offsets, thereby reducing significantly the number of required tools. U.S. Pat. No. 5,611,970 discloses various methods of assuring the two mold sections have the desired rotational alignment with respect to one another. As one example, FIG. 2 illustrates notch 42 in the anterior mold section for engagement with its rotational support member, and detectable indicia 35 on the posterior mold section to detect the rotational position of this mold section.
The present inventors recognized, however, that prior methods sometimes do not provide sufficient control of the rotational positions between the two mold sections. Accordingly, this invention provides more robust and consistent alignment of the two mold sections employed for cast molding toric contact lenses, yet this invention may be implemented in a relatively simple and cost-effective manner.